The present invention relates to a hydraulic system having a hydraulic accumulator for an automatic transmission and more particularly to a hydraulic accumulator that assists with maintaining a hydraulic pressure to allow for immediate engagement of the transmission upon startup for a vehicle that operates with an engine stop-start system.
In order to improve the fuel economy of vehicles that use an internal combustion engine for propulsion, some vehicles employ a stop-start strategy where, when the vehicle is stopped, for example, at a traffic light, the engine is automatically turned off. When the light turns green and the driver releases the brake to apply pressure to the accelerator pedal, the engine is automatically started to allow the vehicle to accelerate. For such vehicles that also have an automatic transmission, a need arises to somehow maintain the hydraulic pressure in the transmission so that the transmission can engage immediately after the engine is automatically restarted, which will allow the vehicle to accelerate without hesitation.
In order to overcome this concern, some employ an electric pump for pressurizing the hydraulic fluid rather than the conventional engine driven pump. While this allows for the maintenance of the hydraulic pressure while the engine is shut off at the stop light, this system may be more costly and require more packaging space than is desired. It also may require an additional battery to power the electric motor. This system may also require continuous operation even when a pressure charge is not required due to a delay between pump motor startup and hydraulic pump prime time.
Another attempt to overcome this concern uses an external gas accumulator to store extra hydraulic fluid outside of the transmission hydraulic circuit. This also has drawbacks in that it may cost more and take up more package space than is desired. In addition, an external gas accumulator may cause a fluid level change in the oil pan during period accumulation (lower fluid level) and discharge (higher fluid level) that create undesirable conditions, such as oil starvation or spin losses, during engagement of transmission clutches.
Another attempted solution is to use a spring-loaded piston accumulator with a mechanical latch. This approach calls for control system redesign to package it internally. It also presents the same issue as a gas accumulator by changing the fluid level in the oil pan during operation. Such a spring loaded accumulator may require one hundred percent capacity fluid charge in order to mechanically latch the accumulator's piston, which may present an issue when frequent stop-start events occur.